1. Field of the Invention
The present invention relates to an image forming apparatus having an electrostatic brush cleaning device.
2. Discussion of the Background
In an image forming apparatus employing electrophotography, un-transferred toner that remains on the surface of an image bearing member after a toner image is transferred therefrom is typically removed by a cleaning device. A cleaning device employing a blade system has been widely used because it demonstrates good cleaning performance with a simple structure in which toner is scraped from the surface of an image bearing member with the blade.
This blade system is successful in reducing occurrence of image flow (image blur) because the mechanical force of scraping the surface of an image bearing member is strong. However, the surface of an image bearing member is easily damaged or abraded by such a strong mechanical force.
Therefore, an electrostatic brush cleaning system using a magnetic brush or a fur brush has been studied as an alternative to the mechanical blade system to remove toner on the surface of an image bearing member using static electricity. In comparison with the blade cleaning system, the electrostatic brushing cleaning system has relatively high cleaning maintenance ability and is particularly advantageous for a high-speed machine.
An image forming apparatus employing an electrophotography system typically transfers toner on the surface of an image bearing member to a transfer medium such as recording medium by applying a bias having a polarity reverse to the regular polarity of the toner attached to the surface of the image bearing member in the transfer process. Therefore, the toner remaining on the surface of the image bearing member after the transfer process includes both toner regularly charged and toner reversely charged. Therefore, when the electrostatic cleaning system is employed, a mechanism that electrostatically retrieves regularly charged toner and reversely charged toner is suitable.
Unexamined published Japanese patent application publication No. (hereinafter referred to as JP-A) H04-330482 describes a technology of using a cleaning device having a first cleaning brush that is positively charged to electrostatically attract negatively charged toner on the surface of the image bearing member and a second cleaning brush that is negatively charged to electrostatically attract positively charged toner on the surface of the image bearing member.
In this technology, the un-transferred toner remaining on the surface of the image bearing member after the transfer process is sufficiently removed by the structure having the first cleaning brush charged with a polarity reverse to that of the toner and the second cleaning brush charged with the same polarity as that of the toner. Therefore, when there is only one cleaning brush or multiple cleaning brushes all of which are charged with the same polarity, the un-transferred toner remaining on the surface of the image bearing member having either one of the regular polarity or the reverse polarity is not substantially removed, which results in deterioration of the image quality.
However, this electrostatic brush cleaning system having such a structure has a relatively low mechanical force to scrape toner off from the surface of the image bearing member in comparison with the mechanical blade cleaning system. Therefore, for example, corona products produced by a charging device are not sufficiently removed, which easily causes image flow (image blur). In addition, once an object (i.e., image bearing member) to be cleaned is scarred (or damaged), the brush cleaning system is known to have a tendency to repetitively trace the scar or damage with its brush fiber.
Therefore, the scar or damage on the image bearing member forms a deep streaky groove, which easily leads to production of abnormal halftone images with white streaks.
To deal with this, JP 2005-17416-A describes an image forming apparatus employing a two-brush system in which one of the cleaning brushes situated on the upstream side relative to the rotation direction of the image bearing member and the other cleaning brush situated on the downstream side relative to the rotation direction of the image bearing member are brought into contact with the image bearing member such that the edge force per filament of the brush of the cleaning brush on the upstream side is greater than that of the cleaning brush on the downstream side.
In addition, JP 2006-251400-A describes an image formation method in which two cleaning brushes are arranged along the rotation direction of the image bearing member and the toner contains complex particles in which inorganic particulates are dispersed and contained in resin particles. In addition, JP 2009-36956-A describes an image forming apparatus in which the cleaning brush (brush 2) situated on the downstream side relative to the rotation direction of the image bearing member is formed of a more rigid material than that forming the cleaning brush (brush 1) situated on the upstream side relative to the rotation direction of the image bearing member, and the brush 2 has a stronger mechanical scraping force than the brush 1.
However, in all of the image forming apparatuses and the image formation methods described above, the brush filaments still damage or scar the surface of the image bearing member, and thus abnormal half tone images having white streaks are easily produced.